﻿<h1>Code Blocks and Identifier Scopes</h1>

<p>
This article will explain the code blocks and identifier scopes in Go.
</p>

<p><i>
(Please note, the definitions of code block hierarchies in this article
are a little different from Go specification.)
</i></p>

<!--
https://github.com/golang/go/issues/7429#issuecomment-282480782
-->

<a class="anchor" id="block"></a>
<h3>Code Blocks</h3>

<div>

In a Go project, there are four kinds of code blocks (also called blocks later):
<ul>
<li>the <b>universe block</b> contains all project source code.</li>
<li>each package has a <b>package block</b> containing all source code,
	excluding the package import declarations in that package.</li>
<li>each file has a <b>file block</b> containing all the source code,
	including the package import declarations, in that file.</li>
<li>generally, a pair of braces <code>{}</code> encloses a <b>local block</b>.
	However, some local blocks aren't enclosed within <code>{}</code>,
	such blocks are called implicit local blocks.
	The local blocks enclosed in <code>{}</code> are called explicit local blocks.
	The <code>{}</code> in composite literals and type definitions don't form
	local blocks.
</li>
</ul>

Some keywords in all kinds of control flows are followed by some implicit code blocks.
<ul>
<li>
	An <code>if</code>, <code>switch</code> or <code>for</code> keyword
	is followed by two nested local blocks. One is implicit, the other is explicit.
	The explicit one is nested in the implicit one.
	If such a keyword is followed by a short variable declaration,
	then the variables are declared in the implicit block.
</li>
<li>
	An <code>else</code> keyword is followed by one explicit or implicit block,
	which is nested in the implicit block following its <code>if</code> counterpart keyword.
	If the <code>else</code> keyword is followed by another <code>if</code> keyword,
	then the code block following the <code>else</code> keyword can be implicit,
	otherwise, the code block must be explicit.
</li>
<li>
	An <code>select</code> keyword is followed by one explicit block.
</li>
<li>
	Each <code>case</code> and <code>default</code> keyword is followed by one implicit block,
	which is nested in the explicit block following its corresponding
	<code>switch</code> or <code>select</code> keyword.
</li>
</ul>

<p>
The local blocks which aren't nested in any other local blocks are called top-level
(or package-level) local blocks. Top-level local blocks are all function bodies.
</p>

<p>
Note, the input parameters and output results of a function are viewed as being
declared in explicit body code block of the function, even if their declarations
stay out of the pair of braces enclosing the function body block.
</p>
</div>


<div>
Code block hierarchies:
<ul>
<li>package blocks are nested in the universe block.</li>
<li>file blocks are also directly nested in the universe block, instead of package blocks.
	(This explanation is different from Go specification and the <code>go/*</code> standard packages.)
</li>
<li>each top-level local block is nested in both a package block and a file block.
	(This explanation is also different from Go specification and the <code>go/*</code> standard packages.)
</li>
<li>a non-top local block must be nested in another local block.</li>
</ul>

<p><i>
(The differences to Go specification are to make the below explanations for identifier shadowing simpler.)
</i></p>

Here is a picture shows the block hierarchies in a program:
<div class="text-center">
	<img src="res/blocks.png" alt="code block hierarchies"></img>
</div>

<p>
</p>
</div>

<p>
Code blocks are mainly used to explain allowed declaration positions and scopes of source code element identifiers.
</p>

<a class="anchor" id="declaration"></a>
<h3>Source Code Element Declaration Places</h3>

<div>
There are six kinds of source code elements which can be declared:
<ul>
<li>package imports.</li>
<li>defined types and type alias.</li>
<li>named constants.</li>
<li>variables.</li>
<li>functions.</li>
<li>labels.</li>
</ul>
<p>
Labels are used in the <code>break</code>, <code>continue</code>, and <code>goto</code> statements.
</p>
</div>

<p>
A declaration binds a non-blank identifier to a source code element (constant, type, variable, function, label, or package).
In other words, in the declaration, the declared source code element is named as the non-blank identifier.
After the declaration, we can use the non-blank identifier to represent the declared source code element.
</p>



<div>
The following table shows which code blocks all kinds of source code elements can be directly declared in:
<table border="1" class="table table-bordered text-center" style="width: auto !important;">
<thead>
	<tr>
	<th class="text-center" valign="bottom" align="center"></th>
	<th class="text-center" valign="bottom" align="center">the universe block</th>
	<th class="text-center" valign="bottom" align="center">package blocks</th>
	<th class="text-center" valign="bottom" align="center">file blocks</th>
	<th class="text-center" valign="bottom" align="center">local blocks</th>
	</tr>
</thead>
<tbody>
	<tr class="active">
	<th scope="row" class="text-center" valign="middle" align="center">predeclared (built-in elements) <sup>(1)</sup></th>
	<td valign="middle" align="center">Yes</td>
	<td></td>
	<td></td>
	<td></td>
	</tr>
	<tr>
	<th scope="row" class="text-center" valign="middle" align="center">package imports</th>
	<td></td>
	<td></td>
	<td valign="middle" align="center">Yes</td>
	<td></td>
	</tr>
	<tr class="active">
	<th scope="row" class="text-center" valign="middle" align="center">defined types and type alias (non-builtin)</th>
	<td></td>
	<td valign="middle" align="center">Yes</td>
	<td valign="middle" align="center">Yes</td>
	<td valign="middle" align="center">Yes</td>
	</tr>
	<tr>
	<th scope="row" class="text-center" valign="middle" align="center">named constants (non-builtin)</th>
	<td></td>
	<td valign="middle" align="center">Yes</td>
	<td valign="middle" align="center">Yes</td>
	<td valign="middle" align="center">Yes</td>
	</tr>
	<tr class="active">
	<th scope="row" class="text-center" valign="middle" align="center">variables (non-builtin) <sup>(2)</sup></th>
	<td></td>
	<td valign="middle" align="center">Yes</td>
	<td valign="middle" align="center">Yes</td>
	<td valign="middle" align="center">Yes</td>
	</tr>
	<tr>
	<th scope="row" class="text-center" valign="middle" align="center">functions (non-builtin)</th>
	<td></td>
	<td valign="middle" align="center">Yes</td>
	<td valign="middle" align="center">Yes</td>
	<td></td>
	</tr>
	<tr class="active">
	<th scope="row" class="text-center" valign="middle" align="center">labels</th>
	<td></td>
	<td></td>
	<td></td>
	<td valign="middle" align="center">Yes</td>
	</tr>
</tbody>
</table>

<p>
<sup>(1)</sup> predeclared elements are documented in <a href="https://golang.org/pkg/builtin/"><code>builtin</code> standard package</a>.
<br/>
<sup>(2)</sup> excluding struct field variables.
</p>

So,
<ul>
<li>
package imports can never be declared in package blocks and local blocks.
</li>
<li>
functions can never be declared in local blocks.
(Anonymous functions can be enclosed in local blocks but they are not declarations.)
</li>
<li>
labels can only be declared in local blocks.
</li>
</ul>

Please note,
<ul>
<li>
	if the innermost containing blocks of two code element declarations are the same one,
	then the names (identifiers) of the two code elements can't be identical.
</li>
<li>
	the name (identifier) of a package-level code element declared in a package must not
	be identical to any package import name declared in any source file in the package.
</li>
<li>
	if the innermost containing function body blocks of two label declarations are the same one,
	then the names (identifiers) of the two labels can't be identical.
</li>
<li>
	the references of a label must be within the innermost function body block containing the declaration of the label.
</li>
<li>
	some special portions in the implicit local blocks in all kinds of control flows have special requirements.
	Generally, no code elements are allowed to be directly declared in such implicit local blocks, except some short variable declarations.
	<ul>
	<li>
		Each <code>if</code>, <code>switch</code> or <code>for</code> keyword can be closely followed by a short variable declaration.
	</li>
	<li>
		Each <code>case</code> keyword in a <code>select</code> control flow can be closely followed by a short variable declaration.
	</li>
	</ul>
</li>
</ul>

<p><i>
(BTW, the <code>go/*</code> standard packages think file code blocks can only contain package import declarations.)
</i></p>
</div>

<p>
The source code elements declared in package blocks but outside of any local blocks are called package-level source code elements.
Package-level source code elements can be named constants, variables, functions, defined types, or type aliases.
</p>

<a class="anchor" id="scope"></a>
<h3>Scopes of Declared Source Code Elements</h3>

<div>
<p>
The scope of a declared identifier means the identifiable range of the identifier (or visible range).
</p>

Without considering identifier shadowing which will be explained in the last section of the current article,
<a href="https://golang.org/ref/spec#Declarations_and_scope">the scope definitions</a> for the identifiers
of all kinds of source code elements are listed below.
<ul>
<li>
	The scope of a predeclared/built-in identifier is the universe block.
</li>
<li>
	The scope of the identifier of a package import is the file block containing the package import declaration.
</li>
<li>
	The scope of an identifier denoting a constant, type, variable, or function
	(but not method) declared at package level is the package block.
</li>
<li>
	The scope of an identifier denoting a method receiver, function parameter,
	or result variable is the corresponding function body (a local block).
</li>
<li>
	The scope of the identifier of a constant or variable declared inside a function
	begins at the end of the specification of the constant or variable
	(or the end of the declaration for a short declared variable)
	and ends at the end of the innermost containing block.
</li>
<li>
	The scope of the identifier of a <a href="type-system-overview.html#type-definition">defined type</a>
	declared inside a function begins at the identifier in the
	specification of the type ends at the end of the innermost containing block.
</li>
<li>
	The scope of the identifier of a <a href="type-system-overview.html#type-alias">type alias</a>
	declared inside a local block begins at the end
	of the declaration of the type and ends at the end of the innermost containing block.
</li>
<li>
	The scope of a label is the body of the innermost function body block containing the label declaration
	but excludes all the bodies of anonymous functions nested in the containing function.
</li>
</ul>
</div>

<p>
Blank identifiers have no scopes.
</p>

<p><i>
(Note, the predeclared <code>iota</code> is only visible in constant declarations.)
</i></p>

<a class="anchor" id="scope-difference-detail"></a>
<div>
You may have noticed the minor difference of identifier scope definitions
between local type definitions and local variables, local constants and local type aliases.
The difference means a defined type may be able to
reference itself in its declaration.
Here is an example to show the difference.
<pre class="line-numbers"><code class="language-go">package main

func main() {
	// var v int = v   // error: v is undefined
	// const C int = C // error: C is undefined
	/*
	type T = struct {
		*T    // error: T uses <T>
		x []T // error: T uses <T>
	}
	*/

	// Following type definitions are all valid.
	type T struct {
		*T
		x []T
	}
	type A [5]*A
	type S []S
	type M map[int]M
	type F func(F) F
	type Ch chan Ch
	type P *P

	// ...
	var s = make(S, 3)
	s[0] = s
	s = s[0][0][0][0][0][0][0][0]

	var m = M{}
	m[1] = m
	m = m[1][1][1][1][1][1][1][1]

	var p P
	p = &p
	p = ***********************p
	***********************p = p
}
</code></pre>

<p>
</p>

And the scope difference between package-level and local declarations:
<pre class="line-numbers"><code class="language-go">package main

// Here the two identifiers at each line are the
// same one. The right ones are both not the
// predeclared identifiers. Instead, they are
// same as respective left one. So the two
// lines both fail to compile.
/*
const iota = iota // error: constant definition loop
var true = true   // error: typechecking loop
*/

var a = b // can reference variables declared later
var b = 123

func main() {
	// The identifiers at the right side in the
	// next two lines are the predeclared ones.
	const iota = iota // ok
	var true = true   // ok
	_ = true

	// The following lines fail to compile, for
	// c references a later declared variable d.
	/*
	var c = d
	var d = 123
	_ = c
	*/
}
</code></pre>
</div>

<a class="anchor" id="shadow"></a>
<h3>Identifier Shadowing</h3>

<div>
<p>
Ignoring labels, an identifier declared in an outer code block can be
shadowed by the same identifier declared in code blocks
nested (directly or indirectly) in the outer code block.
</p>

<p>
Labels can’t be shadowed.
</p>

<p>
If an identifier is shadowed, its scope will exclude the scopes of its shadowing identifiers.
</p>

Below is an interesting example.
The code contains 6 declared variables named <code>x</code>.
A <code>x</code> declared in a deeper block shadows
the <code>x</code>s declared in shallower blocks.
<pre class="line-numbers"><code class="language-go">package main

import "fmt"

var p0, p1, p2, p3, p4, p5 *int
var x = 9999 // x#0

func main() {
	p0 = &x
	var x = 888  // x#1
	p1 = &x
	for x := 70; x < 77; x++ {  // x#2
		p2 = &x
		x := x - 70 //  // x#3
		p3 = &x
		if x := x - 3; x > 0 { // x#4
			p4 = &x
			x := -x // x#5
			p5 = &x
		}
	}

	// 9999 888 77 6 3 -3
	fmt.Println(*p0, *p1, *p2, *p3, *p4, *p5)
}
</code></pre>

<p>
</p>

Another example: the following program prints <code>Sheep Goat</code> instead of <code>Sheep Sheep</code>.
Please read the comments for explanations.

<pre class="line-numbers"><code class="language-go">package main

import "fmt"

var f = func(b bool) {
	fmt.Print("Goat")
}

func main() {
	var f = func(b bool) {
		fmt.Print("Sheep")
		if b {
			fmt.Print(" ")
			f(!b) // The f is the package-level f.
		}
	}
	f(true) // The f is the local f.
}
</code></pre>

<p>
</p>

If we remove the <code>var</code> keyword in the local <code>f</code> declaration,
or modify the above program as the following shown, then it will print <code>Sheep Sheep</code>.

<pre class="line-numbers"><code class="language-go">func main() {
	var f func(b bool)
	f = func(b bool) {
		fmt.Print("Sheep")
		if b {
			fmt.Print(" ")
			f(!b) // The f is also the local f now.
		}
	}
	f(true)
}
</code></pre>

<p>
</p>

For some circumstances, when identifiers are shadowed by variables
declared with short variable declarations,
some new gophers may get confused about whether a variable
in a short variable declaration is redeclared or newly declared.
The following example (which has bugs) shows the famous trap in Go.
Almost every gopher has ever fallen into the trap in the early days of using Go.

<pre class="line-numbers"><code class="language-go">package main

import "fmt"
import "strconv"

func parseInt(s string) (int, error) {
	n, err := strconv.Atoi(s)
	if err != nil {
		// Some new gophers may think err is an
		// already declared variable in the following
		// short variable declaration. However, both
		// b and err are new declareds here in fact.
		// The new declared err variable shadows the
		// err variable declared above.
		b, err := strconv.ParseBool(s)
		if err != nil {
			return 0, err
		}

		// If execution goes here, some new gophers
		// might expect a nil error will be returned.
		// But in fact, the outer non-nil error will
		// be returned instead, for the scope of the
		// inner err variable ends at the end of the
		// outer if-clause.
		if b {
			n = 1
		}
	}
	return n, err
}

func main() {
	fmt.Println(parseInt("TRUE"))
}
</code></pre>

The output:

<pre class="output"><code>1 strconv.Atoi: parsing "TRUE": invalid syntax
</code></pre>

<p>
</p>

<!--
<p>
We can use the <code>go vet -shadow</code> command to list
possible mis-shadowing uses.
</p>

	go get -u golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow
	go vet -vettool=$(which shadow)

https://github.com/golang/go/issues/29260#issuecomment-447694762
-->

<a class="anchor" id="weird-shadowing"></a>

Go only has <a href="keywords-and-identifiers.html#keyword">25 keywords</a>.
Keywords can't be used as identifiers.
Many familiar words in Go are not keywords, such as <code>int</code>, <code>bool</code>,
<code>string</code>, <code>len</code>, <code>cap</code>, <code>nil</code>, etc.
They are just predeclared (built-in) identifiers.
These predeclared identifiers are declared in the universe block,
so custom defined identifiers can shadow them.
Here is a weird example in which many predeclared identifiers are shadowed.
Its compiles and runs okay.
<pre class="line-numbers"><code class="language-go">package main

import (
	"fmt"
)

// Shadows the built-in function identifier "len".
const len = 3
// Shadows the built-in const identifier "true".
var true = 0
// Shadows the built-in variable identifier "nil".
type nil struct {}
// Shadows the built-in type identifier "int".
func int(){}

func main() {
	fmt.Println("a weird program")
	var output = fmt.Println

	// Shadows the package import "fmt".
	var fmt = [len]nil{{}, {}, {}}
	// Sorry, "len" is a constant.
	// var n = len(fmt)
	// Use the built-in cap function instead, :(
	var n = cap(fmt)

	// The "for" keyword is followed by one
	// implicit local code block and one explicit
	// local code block. The iteration variable
	// "true" shadows the package-level variable
	// "true" declared above.
	for true := 0; true < n; true++ {
		// Shadows the built-in const "false".
		var false = fmt[true]
		// The new declared "true" variable
		// shadows the iteration variable "true".
		var true = true+1
		// Sorry, "fmt" is an array, not a package.
		// fmt.Println(true, false)
		output(true, false)
	}
}
</code></pre>

The output:
<pre class="output"><code>a weird program
1 {}
2 {}
3 {}
</code></pre>

<p>
Yes, this example is extreme.
It contains many bad practices.
Identifier shadowing is useful, but please don't abuse it.
</p>

</div>
